For practical purposes, a microscopic (i.e. how many atoms) view of substances is not useful. We need to deal in macroscopic (e.g. how many grams) quantities of elements and compounds. To do this, we introduce the mole.
One mole of a substance is the amount of substance which contains a standard number of particles (atoms, ions or molecules). This standard number is defined as the same number of particles as there are Carbon atoms in 12g of the isotope Carbon-12. This is the Avogadro number 6.022 x 1023 particles.
So 12g of Carbon-12 represents one mole (written 1 mol) of Carbon-12. It contains 6.022 x 1023 Carbon atoms. Similarly one mole of sodium atoms contains 6.022 x 1023 sodium atoms. And half a mol of Neon atoms contains 3.011 x 1023 Neon atoms.
Because atoms of different elements do not have the same mass, one mole of sodium will not be as heavy as one mole of calcium. However, the mass of any substance can easily be related to the number of particles in it, as follows:
• R.A.M. of C = 12 and R.A.M. of He = 4
• 1 C atom is 3 times as heavy as 1 He atom
• 10 C atoms are 3 times as heavy as 10 He atoms
• 1000 C atoms are 3 times as heavy as 1000 He atoms
• 106 C atoms are 3 times as heavy as 106 He atoms
So if a sample of C has 3 times the mass of a sample of He, both samples must contain the same number of atoms.
Why is the mole useful?
If we express the R.A.M. of any element in grams, this must contain the same number of atoms (6.022 x 1023). Th e R.A.M. of any element expressed in grams contains 1mol of that element. This is called the molar mass, and is given the symbol M.
For example, Beryllium (R.A.M. = 9); Argon (R.A.M. = 40). Therefore 9 g of Beryllium and 40 g of Argon contains the same number of atoms (6.022 x 1023). Another example, Silicon (R.A.M. = 28), therefore the molar mass of Si is 28 g mol-1 and is represented by M (Si).
Molecules
The mole concept is equally applicable to molecules. We introduce the relative molecular mass (R.M.M.) which is expressed in grams and represents one mole of the element or compound (i.e. 6.022 x 1023 molecules).
1. Methane is CH4 RMM of methane is RAM of C + 4(RAM H) = 12 + 4x1 = 16 g mol-1 = molar mass of methane.
In 16g methane we can find one mole of methane. And 16g methane contains 6.022 x 1023 methane molecules.
2. Caffeine (C8H10N4O2) and R.M.M. = (8 x 12) + (10 x 1) + (4 x 14) + (2 x 16) = 194
Therefore a mass of 194 g equals 1 mol of caffeine and 194 g mol-1 is the molar mass of caffeine.
3. One mole of oxygen molecules (O2) contains 6.022 x 1023 oxygen molecules which of course contains two moles of oxygen atoms! M (O2). = 16 + 16 = 32
Therefore a mass of 32 equals 1 mol O2 and 32 g mol-1 is the molar mass of O2 represented by M (O2)
Ionic compounds
Th e mole concept is equally applicable to formula units and ions. A relative formula mass (R.F.M.) expressed in grams represents one mole of the compound (i.e. 6.022 x 1023 ‘formula units’).
For example, M (NaCl) = 23 + 35.5 = 58.5
Therefore, 58.5 g equals 1mol of sodium chloride. In other words, 58.5 g mol-1 is the molar mass of NaCl.
Ions
For individual ions, it is common practice to use R.A.M. values
For example, 1 mol Na+ equals 23g and 1 mol Cl- equals 35.5g.
For polyatomic ions, add the RAM together.
• One mole of calcium chloride (CaCl2) contains 1 mole of calcium ions, but two moles of chloride ions. Again, you must state clearly which particles you are referring to when giving the number of moles. How many moles of sodium ions are there in 1 mol sodium Carbonate? We find 2 moles of Na+ ions in 1 mole of Na2CO3.
Molecules
The mole concept is equally applicable to molecules. We introduce the relative molecular mass (R.M.M.) which is expressed in grams and represents one mole of the element or compound (i.e. 6.022 x 1023 molecules).
1. Methane is CH4 RMM of methane is RAM of C + 4(RAM H) = 12 + 4x1 = 16 g mol-1 = molar mass of methane.
In 16g methane we can find one mole of methane. And 16g methane contains 6.022 x 1023 methane molecules.
2. Caffeine (C8H10N4O2) and R.M.M. = (8 x 12) + (10 x 1) + (4 x 14) + (2 x 16) = 194
Therefore a mass of 194 g equals 1 mol of caffeine and 194 g mol-1 is the molar mass of caffeine.
3. One mole of oxygen molecules (O2) contains 6.022 x 1023 oxygen molecules which of course contains two moles of oxygen atoms! M (O2). = 16 + 16 = 32
Therefore a mass of 32 equals 1 mol O2 and 32 g mol-1 is the molar mass of O2 represented by M (O2)
Ionic compounds
Th e mole concept is equally applicable to formula units and ions. A relative formula mass (R.F.M.) expressed in grams represents one mole of the compound (i.e. 6.022 x 1023 ‘formula units’).
For example, M (NaCl) = 23 + 35.5 = 58.5
Therefore, 58.5 g equals 1mol of sodium chloride. In other words, 58.5 g mol-1 is the molar mass of NaCl.
Ions
For individual ions, it is common practice to use R.A.M. values
For example, 1 mol Na+ equals 23g and 1 mol Cl- equals 35.5g.
For polyatomic ions, add the RAM together.
• One mole of calcium chloride (CaCl2) contains 1 mole of calcium ions, but two moles of chloride ions. Again, you must state clearly which particles you are referring to when giving the number of moles. How many moles of sodium ions are there in 1 mol sodium Carbonate? We find 2 moles of Na+ ions in 1 mole of Na2CO3.
